Video Transcript
Which of the following is not
correct about carboxylic acids? (A) They can be detected using
sodium carbonate. (B) They can be prepared by the
oxidation of the corresponding alcohols. (C) They have a lower boiling point
than alcohols that have the same number of carbon atoms. (D) They are more acidic than
alcohols that have the same number of carbon atoms. Or (E) they are less acidic than
mineral acids.
Recall that carboxylic acids are
organic molecules with a carboxyl group as the signature functional group present in
the molecule. The R group varies depending on the
carboxylic acid being discussed. Two examples of carboxylic acids
with different R groups are the well-known two-carbon carboxylic acid called acetic
acid, which is also known as ethanoic acid, and the simplest aromatic carboxylic
acid, benzoic acid. In terms of acid strength,
carboxylic acids are more acidic than alcohols with the same number of carbon atoms,
while mineral acids are more acidic than carboxylic acids.
Recall a strong acid ionizes
completely in solution. In other words, a strong acid like
HA will completely dissociate in water to yield the positively charged hydronium ion
H3O+ and the negatively charged conjugate base A−. If, on the other hand, HA is a weak
acid, it will only exhibit partial ionization. And therefore, the reaction will be
in equilibrium. What this means is that while some
of the weak acid HA will dissociate to generate the products H3O+ and its conjugate
base A−, at the same time, H3O+ and the conjugate base A− will generate the
reactants H2O and the weak acid HA. The reason mineral acids are
stronger acids than carboxylic acids is because mineral acids, like HCl, HNO3, and
the other mineral acids listed here, ionize completely in solution and therefore are
considered strong acids.
The reason carboxylic acids are
stronger acids than alcohols is because of the increased stability of the conjugate
base of the carboxylic acid, otherwise known as the carboxylate anion, due to the
delocalization or spreading of electrons across the carboxyl group. As a result, answer choices (D) and
(E) are both true statements about carboxylic acids. However, the question is asking
which of the following answer choices is not correct about carboxylic acids. Because answer choices (D) and (E)
are correct in describing carboxylic acids, they cannot be the answer for this
question. As it turns out, both answer
choices (A) and (B) are also true statements describing carboxylic acids.
When carboxylic acids react with
sodium carbonate, they produce water, a carboxylate salt, and carbon dioxide. The production of the gaseous
carbon dioxide and the generally visible gas bubbling from the solution makes this a
quick and easy test for the presence of a carboxylic acid molecule. In regards to preparing carboxylic
acids, oxidation of their corresponding alcohols or even corresponding aldehydes
with the strong oxidizing chromate salt, like potassium chromate, in the presence of
sulfuric acid and water for the latter reaction produces the desired carboxylic
acid. Because answer choices (A) and (B)
are both true for carboxylic acids and we are looking for an answer choice that is
not correct about carboxylic acids, we can eliminate (A) and (B) as possible answers
for this question.
This means answer choice (C) is the
incorrect statement because carboxylic acids tend to exhibit higher boiling points
than their corresponding alcohols, not lower boiling points as the answer choice
states. Using acetic acid, also known as
ethanoic acid, and its corresponding alcohol, ethanol, as examples, we can see that
even though they have the same number of carbon atoms, acetic acid, like other
carboxylic acids, demonstrates the ability to form two hydrogen bonds with a second
acetic acid molecule to form an acetic acid dimer. Alcohols, like ethanol, which only
have one hydrogen bonding side on the hydroxy group, cannot form alcohol-based
dimers of comparable intermolecular strength to carboxylic acid dimers.
Dimer formation between carboxylic
acid molecules creates larger molecules that exhibit greater dispersion forces
between carboxylic acid dimers. This in turn results in their
higher boiling point in comparison to their corresponding alcohol because more
energy is required to overcome these attractive forces. For example, the higher boiling
point is observed for acetic acid, which is 118 degrees Celsius, compared to the
boiling point of ethanol, which is 78 degrees Celsius. Therefore, the statement that is
not correct about carboxylic acids is answer choice (C).
